The present invention relates to new two-component polyurethane adhesives of improved flexibility which are particularly suitable for adhering SMC (sheet molding compounds) and metals.
Two-component polyurethane adhesives which are based on a combination of polyurethanes with epoxides have been known for a long time and in many different variations, but have some disadvantages. Two-component polyurethane adhesive systems are described in U.S. Pat. Nos. 4,613,660, and 4,726,868, and published European application Nos. 279,721 and 256,162. In the compositions described in these references, the isocyanate component is reacted with a polyol-epoxide mixture which contains the catalysts. The choice of catalysts which can be used is severely limited because of possible side reactions in order to ensure the storage stability of the mixture. The limited choice of possible catalysts is a disadvantage.
In addition, systems are also known in which the epoxide is contained in the isocyanate component. According to U.S. Pat. No. 4,698,408, one-component is an epoxide-isocyanate mixture and the other component is a mixture of a polyol and an amine, which contains both urethanization and epoxide hardening catalysts. According to U.S. Pat. No. 4,740,539, an epoxide is a constituent of the isocyanate component, whereas an epoxide hardening catalyst is added. According to U.S. Pat. No. 4,740,539, two independent polymer networks (inter-penetrating network) are said to be formed.
However, all the system with isocyanate/epoxide mixtures have the common feature that the epoxides must be as free as possible from hydroxyl functions, so that no side reactions at all impair the storage stability, since mixtures of this type are known to keep for only a limited period and to become resinous rapidly.
Processes for the two-stage production of shaped articles by reaction of an organic polyisocyanate and an epoxide using an alkylating sulphonic acid alkyl ester of methyl iodide or of dimethyl sulphate and if appropriate auxiliaries and additives containing hydroxyl groups are known from German Auslegeschrift 3,644,382. The resulting resins can be employed as adhesives, but do not have the flexibility required for many applications.